Webpack reads your code and finds all module dependencies, for each module. It stores this info in a single tree.
This means you can optimize your dependencies.
Or analyze entire tree after bundling: https://webpack.github.io/analyse/
After version 2.0.0, it will be possible to eliminate dead export code: http://www.2ality.com/2015/12/webpack-tree-shaking.html
Also, it has other killer features like dev server, hot code push and other...
In shorten, Webpack is better.